1. Field of the Invention
The present invention relates generally to digital cellular mobile communications such as GSM (Global System for Mobile communications) systems, and more specifically to mobile radio telephone receiver susceptible to errors introduced to burst signals interleaved over several TDMA (time division multiple access) frames.
2. Description of the Related Art
According to the GSM standard, a 20-ms speech frame of 160 samples is converted into convolutional codes of rate 1/2and block-diagonally interleaved over eight TDMA frames to provide protection against burst errors. At a mobile station site, a CRC (cyclic redundancy check) circuit is provided for performing in error check on de-interleaved, Viterbi-decoded speech samples. If an error is found, an error check bit is supplied to a speech decoder to disable a speech fame by muting its output or replacing the disabled frame with a predetermined speech pattern to prevent noise which would otherwise occur as a result of error bits. Specifically, the CRC circuit performs the error check by producing a 3-bit parity from the 50 most perceptually significant bits of 260 user information bits using the polynomial g(D)=D.sup.3 +D+1. However, if a significant amount of error occurs due to the presence of local obstructions or multipath fades or an interruption of transmission, the speech decoder is likely to falsely treat the transmitted signal as containing normal speech frames with a probability of 12.5% (=(2.sup.47 -1)/2.sup.50 .times.100). Thus, the speech decoder produces unpleasant noise with a predetermined probability when burst errors occur.